Multi-layer magnetic nanoparticles for magnetic recording

ABSTRACT

According to one embodiment, a multi-layer magnetic nanoparticle includes a core; a first magnetic layer deposited on a surface of the core; and a second magnetic layer deposited on a surface of the first magnetic layer, where the core, the first magnetic layer and the second magnetic layer comprise different magnetic anisotropies and/or saturation magnetizations.

BACKGROUND

The present invention relates to data storage systems, and moreparticularly, this invention relates to multi-layer magneticnanoparticles, which may he especially suited for use in magneticrecording media.

In magnetic storage systems, magnetic transducers read data from andwrite data onto magnetic recording media. Data is written on themagnetic recording media by moving a magnetic recording transducer to aposition over the media where the data is to be stored. The magneticrecording transducer then generates a magnetic field, which encodes thedata into the magnetic media. Data is read from the media by similarlypositioning the magnetic read transducer and then sensing the magneticfield of the magnetic media. Read and write operations may beindependently synchronized with the movement of the media to ensure thatthe data can be read from and written to the desired location on themedia.

An important and continuing goal in the data storage industry is that ofincreasing the density of data stored on a magnetic medium. For tapestorage systems, this goal has led to an increase in the track andlinear bit density on recording tape, and a decrease in the thickness ofthe magnetic tape medium.

One approach to achieve higher recording densities in magnetic media isto reduce the size of the recording bits, which typically necessitatesthe design of smaller and smaller components. However, miniaturizationof the recoding bits and components associated therewith, whileeffective, presents several challenges. For instance, as the magneticparticles in a magnetic recording layer become smaller and smaller, themagnetic particles may become thermally unstable, such that thermalfluctuations result in magnetization reversal and the loss of recordeddata. Increasing the magnetic anisotropy of the magnetic particles mayimprove the thermal stability thereof; however, an increase in themagnetic anisotropy requires an increase in the switching field neededto switch the magnetization of the magnetic particles during a writeoperation. Thus, the trilemma associated with magnetic recording relatesto the difficulty in: (1) increasing the media signal to noise ratio(SNR); (2) maintaining media thermal stability; and (3) maintainingmedia write-ability.

BRIEF SUMMARY

According to one embodiment, a multi-layer magnetic nanoparticleincludes a core; a first magnetic layer deposited on a surface of thecore; and a second magnetic layer deposited on a surface of the firstmagnetic layer, where the core, the first magnetic layer and the secondmagnetic layer comprise different magnetic anisotropies and/orsaturation magnetizations.

According to another embodiment, a product includes a magnetic recordingmedium having a substrate and a layer of magnetic nanoparticlesdeposited above the substrate, the magnetic nanoparticles having a core;a first magnetic layer deposited on a surface of the core; and a secondmagnetic layer deposited on a surface of the first magnetic layer, wherethe core, the first magnetic layer and the second magnetic layercomprise different magnetic anisotropies and/or saturationmagnetizations.

Any of these embodiments may be implemented in a magnetic data storagesystem such as a tape drive system, which may include a magnetic head, adrive mechanism for passing a magnetic medium (e.g., recording tape)over the magnetic head, and a controller electrically coupled to themagnetic head,

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VEINS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a multi-layer magnetic nanoparticleaccording to one embodiment.

FIG. 2 is schematic diagram of a product including a magnetic recordingmedium according to one embodiment.

FIG. 3 is a schematic diagram of a simplified tape drive systemaccording to one embodiment.

FIG. 4 is a plot coercivity of a magnetic nanoparticle having a core anda single magnetic shell versus the magnetic field angle, for increasingthicknesses of the single magnetic shell.

FIG. 5 is a plot of the coercivity of a magnetic nanoparticle having acore and a single magnetic shell versus the magnetic field angle, forincreasing saturation magnetization of the single magnetic shell.

FIG. 6 is a plot of the coercivity of a magnetic nanoparticle having acore and one or more magnetic shells versus the magnetic field angle,for varying quantities, thicknesses and/or saturation magnetizations, M,of the magnetic shells.

FIG. 7 is a plot of the ratio of the coercivity of a magneticnanoparticle (He) to that of its core (HeCore), as a function of themagnetic field angle, for varying quantities, thicknesses and/orsaturation magnetizations, M, of one more magnetic shells depositedabove the core.

FIG. 8 is a plot of the ratio of the coercivity of a magneticnanoparticle (He) to that of its core (HeCore) at a magnetic field angleof 60 degrees, for varying thicknesses and/or saturation magnetizationsof one or more magnetic shells deposited above the core.

FIG. 9 is a plot of the coercivity of a magnetic nanoparticle having acore and two magnetic shells, as a function of the magnetic field angle,for varying magnetic anisotropies of the first, intermediate magneticshell deposited directly on the core.

FIG. 10 is a plot of the ratio of the coercivity of a magneticnanoparticle (He) to that of its core (HeCore), as a function of themagnetic field angle, for varying magnetic anisotropies of a first,intermediate magnetic shell deposited directly on the core.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

As also used herein, the term “about” denotes an interval of accuracythat ensures the technical effect of the feature in question. In variousapproaches, the term “about” when combined with a value, refers to plusand minus 10% of the reference value. For example, a thickness of about10 Å refers to a thickness of 10 Å±1 Å. Additionally, the term“substantially” as used in various approaches may refer to within 0 to1% of the reference value.

The following description discloses several preferred embodiments ofmagnetic storage systems, as well as operation and/or component partsthereof.

The storage capacity of a magnetic recording medium may be increased byincreasing the areal density of a magnetic recording layer. However,increasing the areal density of a magnetic recording layer may presentchallenges associated with managing the thermal stability, thewrite-ability and signal to noise ratio thereof.

A particulate magnetic recording layer includes an assembly of magneticnanoparticles. The magnetization orientation of the nanoparticles storesthe recorded information. To increase the recoding density of aparticular magnetic recording layer, the volume of the magneticnanoparticles may be decreased. However, reducing the volume of themagnetic nanoparticles may affect their thermal stability. The thermalstability of a magnetic nanoparticle is given by: K_(u)V/k_(b)T, whereK_(u) denotes the magnetic anisotropy of the nanoparticle, V is thenanoparticle volume, k_(b) denotes the Boltzmann constant, and T denotesthe temperature. Typically, K_(u)v/k_(b)T>˜40, preferably greater than˜60, to avoid thermal decay. To compensate for the reduction in volume,V, of the magnetic nanoparticles, the magnetic anisotropy (K_(a)) of themagnetic nanoparticles may be increased to maintain thermal stability.However, increasing the particle anisotropy results in an increase inthe switching field (i.e. the write field) required to switch themagnetization orientation of the nanoparticles during a write operation.For single phase magnetic nanoparticles, the write field is proportionalto the magnetic anisotropy constant K_(u). Unfortunately, there is alimit to the write field that can be produced by a write transducer. Thelimitation comes from the saturation magnetization of the materials usedto build the write transducer (e.g. maximum magnetic field value forCoFe alloys is 2.4T).

One approach for maintaining the thermal stability and write-ability ofa particulate recording layer, while also increasing the SNR, mayinvolve including core-shell magnetic nanoparticles (i.e. nanoparticleshaving a core encapsulated in a shell) in the particulate recordinglayer. In such an approach, the core and the shell may be coupledthrough an exchange interaction at their interface. Preferably, theshell may have low magnetic anisotropy to assist the magnetizationreversal of the magnetic core. For instance, the magnetization of theshell may preferably react easily to an applied magnetic field and exerta torque on the magnetization of the core, such that the core mayreverse at a smaller applied field than a similar magnetic nanoparticlewithout the shell. The field assist effect of the shell may be larger inapproaches where the shell has a large magnetization saturation.

However, in various approaches, the shell of these core-shell magneticnanoparticles may be susceptible to oxidation. Accordingly, in oneapproach, the shell may include an oxide material (e.g. Fe3O4), whichmay eliminate and/or reduce such oxidation but ultimately result in theshell having a low saturation magnetization. In another approach, anon-magnetic passivation layer (e.g., C) may coat the upper surface ofthe shell, which while also eliminating and/or reducing such oxidationmay nonetheless increase the size of the core-shell magneticnanoparticle and provide with no magnetic advantage (e.g. nomagnetization saturation of the shell to boost the field assist effect).

Accordingly, embodiments disclosed herein may overcome some of theaforementioned drawbacks by providing multi-layer magnetic nanoparticlescomprising a core, and at least two magnetic layers/shells depositedabove the core. In preferred approaches, the core and the at least twomagnetic layers have different magnetic anisotropies and/or saturationmagnetizations to reduce the switching field needed to reverse themagnetization orientation of the core, thereby enabling high densityrecording. In more preferred approaches, the first magnetic layer maycomprise a soft, high magnetic moment intermetallic material. In stillmore preferred approaches, the second magnetic layer may comprise amaterial that is chemically inert, e.g. not susceptible to oxidation.

Following are several examples of general and specific embodiments ofthe multi-layer magnetic nanoparticles disclosed herein.

In one general embodiment, a multi-layer magnetic nanoparticle includesa core; a first magnetic layer deposited on a surface of the core; and asecond magnetic layer deposited on a surface of the first magneticlayer, where the core, the first magnetic layer and the second magneticlayer comprise different magnetic anisotropies and/or saturationmagnetizations.

In another general embodiment, a product includes a magnetic recordingmedium having a substrate and a layer of magnetic nanoparticlesdeposited above the substrate, the magnetic nanoparticles having a core;a first magnetic layer deposited on a surface of the core; and a secondmagnetic layer deposited on a surface of the first magnetic layer, wherethe core, the first magnetic layer and the second magnetic layercomprise different magnetic anisotropies and/or saturationmagnetizations.

Referring now to FIG. 1, a multi-layer magnetic nanoparticle 100 isshown according to one embodiment. As an option, the multi-layermagnetic nanoparticle 100 may be implemented in conjunction withfeatures from any other embodiment listed herein, such as thosedescribed with reference to the other FIGS. Of course, the multi-layermagnetic nanoparticle 100 and others presented herein may be used invarious applications and/or in permutations which may or may not bespecifically described in the illustrative embodiments listed herein.Further, the multi-layer magnetic nanoparticle 100 presented herein maybe used in any desired environment.

As shown in FIG. 1, the multi-layer magnetic nanoparticle 100 includes acore 102, a first magnetic layer 104 deposited directly on a surface ofthe core 102, and a second magnetic layer 106 deposited directly on asurface of the first magnetic layer 104. As particularly shown in FIG.1, the first magnetic layer 104 substantially encapsulates the core 102,and the second magnetic layer 106 substantially encapsulates the firstmagnetic layer 104. In preferred approaches, the shape of the core 102,the first magnetic layer 104, and/or the second magnetic layer 106, maybe spherical. However, in other approaches, the shape of the core 102,the first magnetic layer 104, and/or the second magnetic layer 106, maybe non-spherical, tubular, irregular, etc. In more approaches, theoverall shape of the magnetic nanoparticle 100 may be spherical,columnar, tubular, in the form of a wire, irregular, etc.

In one embodiment, the core 102 may include a material selected from agroup consisting of FePt (L10), FePd, CoPt, NdFeB, SmCo, BaFeO, andcombinations thereof. In another embodiment, the core 102 may include ahard magnetic material. In yet another embodiment, the core 102 may havea magnetic anisotropy between about 1e⁶ crg/cc and 1e⁸ crg/cc. In stillanother embodiment, the core 102 may have a saturation magnetizationranging from about 250 emu/cc to about 2400 emu/cc. In a furtherembodiment, the core 102 may have a diameter, d_(c), between about 3 toabout 15 nm. Diameters noted herein refer to outer diameters unlessotherwise specified.

The first magnetic layer 104, according to one embodiment, may includean intermetallic material selected from a group consisting of: Fe, Co, aFe alloy, a Co alloy, and combinations thereof. According to anotherembodiment, the first magnetic layer 104 may be a soft magnetic layer.According to yet another the first magnetic layer 104 may have amagnetic anisotropy less than 1e⁶ erg/cc. According to a furtherembodiment, the first magnetic layer 104 may have a magnetic anisotropythat is substantially zero. According to an additional embodiment, thefirst magnetic layer 104 may have a magnetic anisotropy that is lowerthan the core 102. According to some embodiments, the first magneticlayer 104 may have a saturation magnetization that is between about 200emu/cc and about 2400 emu/cc. According to more embodiments, the firstmagnetic layer 104 may have a diameter, d₁, between about 0.5 nm andabout 5 nm.

In one embodiment, the second magnetic layer 106 includes a materialthat is chemically stable, e.g. does not easily oxidize. Thus, inapproaches where the second magnetic layer 106 includes a chemicalstable/inert material, the second magnetic layer 106 may act as apassivation layer for the innermost layers (i.e. the core 102 and thefirst magnetic layer 104). The second magnetic layer 106 may alsoinclude, in another embodiment, a magnetic oxide. In another embodiment,the second magnetic layer 106 may include at least one of Fe₃O₄, andCoFe₂O₄. In yet another embodiment, the second magnetic layer 106 may bea soft magnetic layer. In still another embodiment, the second magneticlayer 106 may have a magnetic anisotropy less than 1e⁶ erg/cc. Infurther embodiments, the second magnetic recording layer 106 may have amagnetic anisotropy that is substantially zero. In more embodiments, thesecond magnetic layer 106 may have a magnetic anisotropy that issubstantially equal to or lower than the first magnetic recording layer104. In yet more embodiments, the second magnetic layer 106 may have asaturation magnetization that is between about 200 emu/cc and about 2400emu/cc. In even more embodiments, the second magnetic layer 106 may havea diameter, d₂, between about 0.5 nm and about 5 nm.

In preferred embodiments, the core 102 has a magnetic anisotropy that ishigher/greater than the magnetic anisotropy of the first magnetic layer104 and/or the second magnetic layer 106. In one particular approach,the magnetic anisotropy of the core 102 may be higher/greater than themagnetic anisotropy of the first magnetic layer 104 and the secondmagnetic layer 106, where the magnetic anisotropies of the first andsecond magnetic layer 104, 106 may be substantially equal, andpreferably about zero.

In another approach, the magnetic anisotropy of the core 102 may behigher/greater than the magnetic anisotropy of the first magnetic layer104, and the magnetic anisotropy of the first magnetic layer 104 may behigher/greater than the second magnetic layer 106. Thus, in thisapproach, the magnetic anisotropy of the core 102 and the first andsecond magnetic layers 104, 106 decreases in a direction extendingoutward from the core 102 to the second magnetic layer 106.

In other embodiments, the core 102, the first magnetic layer 104, andthe second magnetic layer 106 may each comprise a different magneticanisotropy and/or saturation magnetization.

In additional embodiments, the multi-layer magnetic nanoparticle 100 mayinclude one or more additional magnetic layers deposited above a surfaceof the second magnetic layer 106. For instance, in such approaches, themulti-layer magnetic nanoparticle 100 may include a third magnetic layerdeposited on a surface of the second magnetic layer 106, a fourthmagnetic layer deposited on a surface of the third magnetic layer, andso on.

In various approaches, each of the above referenced one or moreadditional magnetic layers may have a magnetic anisotropy that is lowerthan the magnetic anisotropy of the core 102, the first magnetic layer104 and/or the second magnetic layer 106. In preferred approaches, themagnetic anisotropy of the core 102, the first and second magneticlayers 104, 106, and the one or more additional magnetic layersdecreases in a direction extending outward from the core 102 to theoutmost additional layer. In more approaches, the outmost layer of theone or more additional magnetic layer may include a material that ischemically inert/stable, e.g. does not easily oxidize.

In further embodiment, the diameter of the multi-layer magneticnanoparticle 100 may be between about 5 and about 20 nm.

In other embodiments, a plurality of the multi-layer magneticnanoparticles 100 may be dispersed/embedded in a suitable matrix. Invarious approaches, the suitable matrix may be selected based on theparticular application (e.g. magnetic recording applications,semiconductor applications, optoelectronic applications, etc.). Examplesof a suitable matrix may include, but are not limited to, bindermaterials, ceramics, gels, semiconductors, plastics, reinforcedplastics, etc. and other such materials as would be recognized by onehaving skill in the art upon reading the present disclosure. Moreover, aplurality of the magnetic nanoparticles 100 may be dispersed/embedded asuitable matrix to form a thin film (e.g., a magnetic recording layer)as described in more detail herein.

In more embodiments, formation or the multi-layer magnet nanoparticle100 may be achieved via chemical approaches in a solution, chemicalapproaches in a sol-gel, vacuum deposition, evaporation, etc. and othersuitable methods as would be understood by one having skill in the artupon reading the present disclosure.

Referring now to FIG. 2, a product 200 including a magnetic recordingmedium 202 is shown according to one embodiments an option, the product200 may be implemented in conjunction with features from any otherembodiment listed herein, such as those described with reference to theother FIGS. Of course, the product 200 may be used in variousapplications and/or in permutations which may or may not be specificallydescribed in the illustrative embodiments listed herein. Further, theproduct 200 presented herein may be used in any desired environment.

As shown in FIG. 2, the magnetic recording medium 202 includes asubstrate 204, which may include glass, ceramic materials, glass/ceramicmixtures, AlMg, silicon, silicon-carbide, or other substrate materialsuitable for use in magnetic recording media as would be recognized byone having skill in the art upon reading the present disclosure. In oneoptional approach, the magnetic recording medium 202 may include anoptional adhesion layer (not shown in FIG. 2) above the substrate 204 topromote coupling of layers formed thereabove.

As also shown in FIG. 2, the magnetic recording medium 202 may includeone or more underlayers 206 as known in the art to promote datarecording in the magnetic recording layer 208 and/or ordered formationof the magnetic recording layer 208. The magnetic recording layer 208 ispositioned above the one or more underlayers 206 and is configured torecord data therein.

The magnetic recording layer 208 includes a plurality of magneticnanoparticles 210. In various approaches, each of the plurality ofmagnetic nanoparticles may be the multi-layer magnetic nanoparticle 100described in FIG. 1.

With continued reference to FIG. 2, the spacing between the magneticnanoparticles 210 may be substantially uniform, in preferred approaches.An intermediate material (e.g. a matrix, a segregant as known in theart) 212 may surround the magnetic nanoparticles 210 to isolate themagnetic nanoparticles 210 and/or maintain the substantially uniformspacing between the magnetic nanoparticles 210. In various approaches,the intermediate material 212 may include an organic compound or aninorganic compound.

In some approaches, the magnetic recording layer 208 may include amonolayer of the magnetic nanoparticles 210, as shown in FIG. 2. Inother approaches, the magnetic recording layer may include one or morelayers of the magnetic nanoparticles 210. The nanoparticles preferablyhave a substantially ordered arrangement, but may be randomly spaced,etc.

In additional approaches, formation of the magnetic recording layer 208,which has the plurality of magnetic nanoparticles 210 dispersed within,may be formed above the underlayers 206 via a spin coating method; amethod involving site selective binding of the magnetic nanoparticles210 (e.g. spin coating combined with temperature, magnetic or electricalfield gradients to control the placement of the magnetic nanoparticles210) and overcoating the magnetic nanoparticles 210 with theintermediate material 212; vacuum deposition; application of a liquiddispersion comprising the magnetic nanoparticles 212 in a solvent to asurface upon which the magnetic recording layer 208 is to be formed, andevaporating the solvent to form a layer of the magnetic nanoparticles212 upon said surface; etc. and other such methods as would beunderstood by one having skill in the art upon reading the presentdisclosure.

As further shown in FIG. 2, the magnetic recording medium 202 may alsoinclude a protective overcoat 214 as known in the art that is configuredto protect the magnetic recording layer from wear, corrosion, etc. It isimportant to note that the magnetic recording medium 202 may includemore or less layers than those shown in FIG. 2.

In various approaches, the magnetic recording medium may be alongitudinal recording medium, a perpendicular magnetic recording media,a patterned magnetic recording medium (e.g. a discrete track medium, abit patterned recording medium), etc. In additional approaches, themagnetic recording medium may be a magnetic tape, a magnetic disk, amagnetic card, etc.

In approaches where the magnetic recording medium 202 may be a magnetictape, the product 200 may also include a tape drive, such as thatdescribed below.

FIG. 3 illustrates a simplified tape drive 300, which may be employed inthe context of the present invention. While one specific implementationof a tape drive is shown in FIG. 3, it should be noted that theembodiments described herein may be implemented in the context of anytype of tape drive system.

As shown, a tape supply cartridge 320 and a take-up reel 321 areprovided to support a magnetic tape 322. One or more of the reels mayform part of a removable cartridge and are not necessarily part of thetape drive 300. The tape drive may further include drive motor(s) todrive the tape supply cartridge 320 and the take-up reel 321 to move thetape 322 over a tape head 326 of any type. This tape head may include anarray of readers, writers, or both.

Guides 325 guide the magnetic tape 322 across the tape head 326. Thistape head 326 is in turn coupled to a controller 328 via a cable 330.The controller 328, may be or include a processor and/or any logic forcontrolling any subsystem of the drive 300. For example, the controller328 typically controls head functions such as servo following, datawriting, data reading, etc. The controller 328 may operate under logicknown in the art, as well as any logic disclosed herein. The controller328 may be coupled to a memory 336 of any known type, which may storeinstructions executable by the controller 328. Moreover, the controller328 may be configured and/or programmable to perform or control some orall of the methodology presented herein. Thus, the controller may beconsidered configured to perform various operations by way of logicprogrammed into a chip; software, firmware, or other instructions beingavailable to a processor; etc. and combinations thereof.

The cable 330 may include read/write circuits to transmit data to thetape head 326 to be recorded on the magnetic tape 322 and to receivedata read by the tape head 326 from the magnetic tape 322. An actuator332 controls position of the tape head 326 relative to the magnetic tape322.

An interface 334 may also be provided for communication between the tapedrive 300 and a host (integral or external) to send and receive the dataand for controlling the operation of the tape drive 300 andcommunicating the status of the tape drive 300 to the host, all as willbe understood by those of skill in the art.

EXAMPLES

The following non-limiting examples several embodiments of multi-layermagnetic particles, such as those disclosed herein. It is important tonote that the following examples are for illustrative purposes only anddo not limit the invention in anyway. It should also be understood thatvariations and modifications of these may he made by those skilled inthe art without departing from the spirit and scope of the invention.

Example 1

Example 1 concerns a simulation of a magnetic nanoparticle having a coreand a single magnetic shell surrounding the core, where the core isexchange-coupled with the single magnetic shell. In this Example, thecore has a 5 nm diameter, a magnetic anisotropy of 2.5e⁷ erg/cc, and asaturation magnetization of 1000 emu/cc. The magnetic properties of thecore (e.g. the magnetic anisotropy and saturation magnetization) in thesimulated magnetic nanoparticle of Example 1 are similar to that ofFePt.

FIG. 4 illustrates the evolution of the magnetic nanoparticle'scoercivity (He), as a function of field angle relative to the uniaxial,easy axis of the core, for increasing thicknesses of the single magneticshell. Magnetic coercivity the field required to reverse themagnetization of the nanoparticle. The field angle corresponds to theangle at which the magnetic field is applied relative to the uniaxial,easy axis of the core. The parameters (e.g. shell thickness anddiameter) corresponding to each of the curves illustrated in FIG. 4 areshown in Table 1 below.

TABLE 1 Curve Core diameter (nm) Shell thickness/diameter (nm) A₁ 5 0/0B₁ 5 1.25/6.25 C₁ 5 2.5/7.5 D₁ 5  5/10

As shown in FIG. 4, increasing the thickness of the single magneticshell reduces coercivity of the magnetic nanoparticle. In preferredapproaches, the coercivity of the magnetic nanoparticle may be less thanor equal to 20 kOe for practical magnetic recording purposes. Whiteincreasing the thickness of the single magnetic shell may better assistthe magnetization reversal of the magnetic nanoparticle, such increasemay also come at the expense of larger particle volume. Thus, thethickness of the single magnetic shell may need to be minimized,otherwise the overall gain in SNR due to a reduction of the nanoparticlevolume cannot be met.

Accordingly, where the thickness of the magnetic shell may be minimal toreduce the overall nanoparticle volume, the magnetic shell should have alarge saturation magnetization to optimally assist magnetizationreversal. FIG. 5 illustrates the evolution of the magneticnanoparticle's coercivity, as a function of field angle, for increasingsaturation magnetization of the single magnetic shell having a magneticanisotropy of zero, a thickness of 1.25 nm, and an outer diameter of6.25 nm. The parameters (e.g. saturation magnetization (M) of the shell)corresponding to each of the curves illustrated in FIG. 5 are shown inTable 2 below.

TABLE 2 Curve Shell saturation magnetization, M (emu/cc) A₂ N/A (noshell) B₂ 250 C₂ 750 D₂ 1000

As shown in FIG. 5, increasing the saturation magnetization of thesingle magnetic shell also reduces coercivity of the magneticnanoparticle.

Example 2

Example 2 concerns a simulation of a magnetic nanoparticle having a coreand one or more magnetic shells surrounding the core, where the core isexchange-coupled with the one or more magnetic shells. In this Example,the core has a 5 nm diameter, a magnetic anisotropy of 2.5e⁷ erg/cc, anda saturation magnetization of 1000 emu/cc.

FIG. 6 illustrates the evolution of the magnetic nanoparticle'scoercivity as a function of field angle relative the uniaxial, easy axisof the core, for varying quantities, thicknesses and/or saturationmagnetizations, M, of magnetic shells, FIG. 7 illustrates the ratio ofthe magnetic nanoparticle's coercivity (He) to that of the core(HeCore), as a function of field angle, for varying quantities,thicknesses and/or saturation magnetizations, M, of magnetic shells. Therelevant and/or varying parameters (e.g. the quantity of each shell, thesaturation magnetization (M) of each shell, and total nanoparticlediameter) corresponding to each of the curves illustrated in FIGS. 6 and7 are shown in Table 3 below. It is important to note that each shellpresented in curves A₃-I₃ described below has a magnetic anisotropy ofzero erg/cc. Moreover, curves F₃-I₃ are only plotted over a subset ofthe field angles as compared to curves A₃-E₃. Finally, it is alsoimportant to note, that the first magnetic shell (shell 1)surrounds/encapsulates the core, shell 2 surrounds/encapsulates shell 2,and so on.

TABLE 3 Shell 1 thickness Shell 2 thickness (nm)/ Total nanoparticleCurve (nm)/M (emu/cc) M (emu/cc) diameter A₃ N/A N/A 5 B₃ 0.5/250 N/A 6C₃ 1.5/500 1.0/non magnetic 10 D₃ 1.5/750 1.0/non magnetic 10 E₃ 1.5/1000 1.0/non magnetic 10 F₃ 2.5/250 N/A 10 G₃ 1.5/500 1.0/250 10 H₃1.5/750 1.0/250 10 I₃  1.5/1000 1.0/250 10

FIGS. 6 and 7 illustrate the advantage (i.e. the reduction in themagnetic nanoparticle's coercivity) of adding a second, outer magneticshell having a relatively low moment/saturation magnetization ascompared to having only one magnetic shell. For instance, the magneticnanoparticles described in curves G₃-I₃, which have a first,intermediate magnetic shell (shell 1) with a high moment and a thinsecond, outer magnetic shell (shell 2) with a low moment, achieve agreater reduction in the nanoparticle's coercivity as compared to themagnetic nanoparticle described in curve F₃, which has only one, lowmoment magnetic shell with an equivalent thickness (e.g. a thicknessequivalent to the sum of the thicknesses of shells 1 and 2 in curvesG₃-I₃). In addition, FIGS. 6 and 7 illustrate the advantage of adding asecond outer magnetic shell having a relatively low moment/saturationmagnetization as compared to having an equivalently thick non-magneticprotective shell. For instance, the magnetic particles described incurves G₃-I₃ achieve a greater reduction in the nanoparticle'scoercivity as compared to the magnetic nanoparticles described in curvesC₃-E₃.

Example 3

Example 3 concerns a simulation of a magnetic nanoparticle having a coreand one or more magnetic shells surrounding the core, where the core isexchange-coupled with the one or more magnetic shells. In this Example,the core has a 5 nm diameter, a magnetic anisotropy of 2.5e⁷ erg/cc, anda saturation magnetization of 1000 emu/cc,

FIG. 8 illustrates the ratio of the magnetic nanoparticle's coercivity(He) to that of the core (HeCore), at a field angle of 60 degreesrelative the uniaxial, easy axis of the core, for thicknesses and/orsaturation magnetizations of the one or more shells. The relevant and/orvarying parameters (e.g. the quantity of each shell, the saturationmagnetization (M) of each shell, and total nanoparticle diameter)corresponding to each of the curves illustrated in FIG. 8 are shown inTable 4 below. It is important to note that each shell present in curvesA₄-E₄ described below has a magnetic anisotropy of zero erg/cc.Moreover, it is also important to note, that the first magnetic shell(shell 1) surrounds/encapsulates the core, shell 2surrounds/encapsulates shell 2, shell 3 surrounds/encapsulates shell 2,and so on.

TABLE 4 Shell 1 thickness Shell 2 thickness (nm)/ Total nanoparticleCurve (nm)/M (emu/cc) M (emu/cc) diameter A₄ N/A N/A 5 B₄ 1.5/1000 N/A 8C₄ 1.5/1000 1.0/250 10 D₄ 2.0/1000 0.5/250 10 E₄ 2.5/1000 N/A 10

FIG. 8 illustrates that a magnetic nanoparticle having a thin outer,magnetic shell (or a thin outer, effective magnetic shell) isadvantageous for reducing the magnetic coercivity of the core. However,in various approaches, reducing the thickness of the outer magneticshell may also lead to an increased risk of oxidation of the outermagnetic shell and, possibly, the inner magnetic shells as welt.Accordingly, in some approaches, the thickness of the outer magneticshell may be selected so as to reduce the coercivity of the core, yetstill passivate the inner magnetic shells and/or prevent oxidationthereof.

Example 4

Example 4 concerns a simulation of a magnetic nanoparticle having acore, a first magnetic shell surrounding the core, and second magneticshell surrounding the first magnetic shell, where the core isexchange-coupled with the two magnetic shells. In this Example, the corehas a 5 nm diameter, a magnetic anisotropy of 2.5e⁷ erg/cc, and asaturation magnetization of 1000 emu/cc. The first magnetic shell (shell1) also has a saturation magnetization of 1000 emu. In addition, thesecond magnetic shell (shell 2) has a magnetic anisotropy of zeroerg/cc, and a saturation magnetization of 250 emu/cc.

FIG. 9 illustrates the magnetic nanoparticle's coercivity (He), as afunction of field angle relative the uniaxial, easy axis of the core,for varying magnetic anisotropies of the first magnetic shell. FIG. 10illustrates the ratio of the magnetic nanoparticle's coercivity (He) tothat of the core (HeCore), as a function of field angle relative theuniaxial, easy axis of the core, for varying magnetic anisotropies ofthe first magnetic shell. The relevant and/or varying parameters (e.g.the magnetic anisotropies) corresponding to each of the curvesillustrated in FIGS. 9 and 10 are shown in Table 5 below.

TABLE 5 Curve Shell 1: magnetic anisotropy (erg/cc) A₅ n/a (no first orsecond shell) B₅ 1e⁴ C₅ 1e⁵ D₅ 1e⁶ E₅ 5e⁶ F₅ 1e⁷ G₅ 2.5e⁷  

In preferred approaches, shells 1 and 2 both have low to zero magneticanisotropies, as smaller magnetic anisotropies may better assist themagnetization reversal of the hard magnetic core. However, as shown inFIGS. 10 and 11, a magnetic nanoparticle including a first magneticshell with some degree of magnetic anisotropy (e.g. between about 1e4 toabout 2.5e7) and a second magnetic shell having a lower magneticanisotropy than the first magnetic shell may still be advantageous (e.g.in terms of reducing the magnetic nanoparticle's coercivity) as comparedto a magnetic nanoparticle having solely a core and/or a first magneticlayer. In more preferred approaches, there is a decreasing gradient inthe magnetic anisotropy ranging from the core to the outermost magneticshells (where there are preferably at least two magnetic shells).

It will be clear that the various features of the foregoing systemsand/or methodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

It will be further appreciated that embodiments of the present inventionmay be provided in the form of a service deployed on behalf of acustomer.

The inventive concepts disclosed herein have been presented by way ofexample to illustrate the myriad features thereof in a plurality ofillustrative scenarios, embodiments, and/or implementations. It shouldbe appreciated that the concepts generally disclosed are to beconsidered as modular, and may be implemented in any combination,permutation, or synthesis thereof. In addition, any modification,alteration, or equivalent of the presently disclosed features,functions, and concepts that would be appreciated by a person havingordinary skill in the art upon reading the instant descriptions shouldalso be considered within the scope of this disclosure.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A multi-layer magnetic nanoparticle, comprising:a core; a first magnetic layer deposited on a surface of the core; and asecond magnetic layer deposited on a surface of the first magneticlayer, wherein the core, the first magnetic layer and the secondmagnetic layer comprise different magnetic anisotropies and/orsaturation magnetizations.
 2. The multi-layer magnetic nanoparticle asrecited in claim 1, wherein the core has a diameter between about 3 toabout 13 nm.
 3. The multi-layer magnetic nanoparticle as recited inclaim 1, wherein the core comprises a material selected from a groupconsisting of: FePt, FePd, CoPt, NdFeB, SmCo, BaFeO, and combinationsthereof.
 4. The multi-layer magnetic nanoparticle as recited in claim 1,wherein the first magnetic layer has a diameter between about 0.5 nm andabout 15 nm.
 5. The multi-layer magnetic nanoparticle as recited inclaim 1, wherein the first magnetic layer comprises an intermetallicmaterial selected from a group consisting of: Fe, Co, a Fe alloy, a Coalloy, and combinations thereof.
 6. The multi-layer magneticnanoparticle as recited in claim 1, wherein the second magnetic layercomprises a material that is chemically stable.
 7. The multi-layermagnetic nanoparticle as recited in claim 1, wherein the second magneticlayer comprises a magnetic oxide.
 8. The multi-layer magneticnanoparticle as recited in claim 1, wherein the second magnetic layercomprises at least one of Fe₃O₄, and CoFe₂O₄.
 9. The multi-layermagnetic nanoparticle as recited in claim 1, wherein the magneticanisotropy of the core is higher than the magnetic anisotropy of thefirst magnetic layer and the magnetic anisotropy of the second magneticlayer.
 10. The multi-layer magnetic nanoparticle as recited in claim 9,wherein the magnetic anisotropy of the first magnetic layer is higherthan the magnetic anisotropy of the second magnetic layer.
 11. Themulti-layer magnetic nanoparticle as recited in claim 1, wherein themagnetic anisotropy of the core is between about 1e⁶ erg/cc and 1e⁸erg/cc.
 12. The multi-layer magnetic nanoparticle as recited in claim 1,wherein the magnetic anisotropy of the first magnetic layer and/or themagnetic anisotropy of the second magnetic layer is less than 1e⁶erg/cc.
 13. The multi-layer magnetic nanoparticle as recited in claim 1,wherein the first magnetic layer and/or the second magnetic layer hassubstantially no magnetic anisotropy.
 14. The multi-layer magneticnanoparticle as recited in claim 1, wherein the saturation magnetizationof the first magnetic layer and/or the second magnetic layer is betweenabout 200 emu/cc and about 2400 emu/cc.
 15. The multi-layer magneticnanoparticle as recited in claim 1, further comprising one or moreadditional layers deposited above a surface of the second magneticlayer, wherein the one or more additional layer have a magneticanisotropy that is lower than the magnetic anisotropy of the core, thefirst magnetic layer and/or the second magnetic layer.
 16. A product,comprising: a magnetic recording medium having a substrate and a layerof magnetic nanoparticles deposited above the substrate, the magneticnanoparticles comprising: a core; a first magnetic layer deposited on asurface of the core; and a second magnetic layer deposited on a surfaceof the first magnetic layer, wherein the core, the first magnetic layerand the second magnetic layer comprise different magnetic anisotropiesand/or saturation magnetizations.
 17. The product as recited in claim16, wherein the magnetic nanoparticles are dispersed in a binder. 18.The product as recited in claim 16, wherein the magnetic nanoparticleshave a substantially uniform diameter between about 5 nm to about 20 nm.19. The product as recited in claim 16, wherein the magnetic anisotropyof the core is higher than the magnetic anisotropy of the first magneticlayer, and the magnetic anisotropy of the first magnetic layer is higherthan the magnetic anisotropy of the second magnetic layer.
 20. Theproduct as recited in claim 16, wherein the magnetic anisotropy of thecore is between about 1e⁶ erg/cc and 1e⁸ erg/cc, wherein the magneticanisotropy of the first magnetic layer and/or the magnetic anisotropy ofthe second magnetic layer is less than 1e⁶ erg/cc.